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Selfing, Outcrossing, and Mixed Mating in the Freshwater Snail Physa heterostropha: Lifetime Fitness and Inbreeding Depression
Amy R. Wethington and Robert T. Dillon, Jr.
Vol. 116, No. 3 (Summer, 1997), pp. 192-199
Stable URL: http://www.jstor.org/stable/3226896
Page Count: 8
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The purpose of this investigation was to estimate inbreeding depression and self-fertilization depression from a comparison of self-fertilizing and outcrossing pulmonate snails, Physa heterostropha. We monitored lifetime survival and fecundity in 26 paired snails from three isofemale lines, using recessive alleles at two complementing albino loci to verify out-crossing. While demonstrably holding allosperm reserves, 10 of these individuals bore self-fertilized progeny at frequencies ranging from 1% to 35%. We designated these 10 individuals "mixed maters," leaving 16 individuals in our sample as more strictly outcrossers. Mixed maters and outcrossers produced comparable numbers of viable embryos per parent, and their embryos displayed negligible differences in survival to 7-14 days post-hatch. Outcrossing and mixed groups displayed nearly identical values for net reproductive rate (808 and 848 offspring/ parent, respectively) and intrinsic rate of natural increase (2.86 and 2.82 per four weeks). Thus in the benign environment of artificial culture, the low levels of self-fertilization occasionally displayed by the previously inseminated snails seem adaptively neutral. The fitness of both the outcrossers and mixed maters greatly exceeded that of 15 individuals isolated as juveniles and hence forced to self-fertilize entirely (R=286, r=1.57). There were no significant differences among selfers, outcrossers, and mixed-maters with respect to parental survival. But selfing parents produced fewer eggs over their lifetimes, higher proportions of apparently infertile eggs, and embryos of much-reduced survival. The estimated inbreeding depression of 0.353, together with an overall self-fertilization depression of 0.646, suggests strong selective pressure against self-fertilization, and evolution toward at least partial outcrossing.
Invertebrate Biology © 1997 American Microscopical Society